Over the years there have been a number of pumps developed for infusion of medical solutions to patients. Such pumping of fluids has been routinely accomplished through a wide variety of well known pumping mechanisms. In the administration of fluids to a patient, it is desirable that the pump be of the "non-wetting" variety, such as that exemplified by the well known peristaltic pump. A peristaltic pump is a type of pump which uses wave-like motion against the walls of a flexible tube containing the fluid being pumped. The non-wetting-type pump is particularly useful in hospital and medical situations in that the fluid being pumped is not subject to contamination through direct contact with the component parts of the pump. In like fashion, if corrosive fluids are being pumped there is no direct contact of corrosive fluid with component parts of the pump.
Another desirable characteristic of pumping mechanisms in general is for the pump to deliver fluid at a rate which remains reasonably constant. In other words, throughout the pumping cycle, the rate of flow should remain even, without any surges or significant variations in the flow rate.
Peristaltic pumps of the non-wetting variety are basically one of two types, namely rotary peristaltic pumps and linear peristaltic pumps. A disadvantage of rotary peristaltic pumps, however, is that they have relatively poor efficiency. In addition, they impose high shear and tension stresses on the tubing which is used to convey the fluid. Another disadvantage is that because of the high forces typically produced by rotary peristaltic pumps, the tubing eventually experiences spalling of its inner walls. There is also, after a period of time, permanent plastic deformation, or "set", of the tubing. In other words, its normally circular cross section becomes flattened into a more oval shape.
Linear peristaltic pumps, in comparison, typically use reciprocating parts to provide peristaltic action against the flexible tube in order to move the fluid through the tube. Such peristaltic pumps consist of a plurality of reciprocating pumping fingers, typically twelve (12), that are sequentially urged against the tube to occlude adjacent segments of tubing in wave-like action. Although linear peristaltic pumps overcome some of the above-stated disadvantages associated with rotary peristaltic pumps, they do so at considerable added cost and with the greater complexity added by the mechanism needed to properly synchronize twelve (12) pumping fingers. Since the pumping fingers are urged to sequentially occlude adjacent segments of tubing, the crushing forces imposed on the tubing and fluid are comparable to those encountered with rotary peristaltic pumps. There is less damage, however, than that caused by rotary peristaltic pumps, since the occlusion forces are localized to the area beneath each finger rather than being applied in movement along the whole length of the tubing. Nonetheless, even with a linear peristaltic pump, there is still some damage such as plastic deformation of the tubing. As a consequence, the structural integrity of the tube carrying the fluid is compromised and as the tubing assumes a progressively more oval cross-sectional shape, the volume and flow rate of the fluid delivered in each pumping cycle is affected.
Furthermore, in order to smooth the pumping transition from one cycle to the next, some linear peristaltic pumps have what is called a "wrap" cycle. During a "wrap" cycle, the motor driving the pump is accelerated to quickly move the upstream finger into occlusion. Thereafter, the motor can resume normal speed to sequentially squeeze and occlude adjacent portions of the tube in its wave-like cycle action. Incorporating this "wrap" cycle can require use of a relatively large and expensive motor with high acceleration capability. Also, because fluid is not delivered during this "wrap" cycle, most linear peristaltic pumps use many fingers (e.g. twelve (12) additional pumping fingers, as mentioned earlier) to minimize the proportionate time of the "wrap" cycle. Maintaining proper alignment and relational movement between such a plurality of fingers also deteriorates the reliability of operation of the device and increases manufacturing costs.
Accordingly, it is an object of the present invention to provide a peristaltic pump of the non-wetting type which is simple and efficient in operation. It is another object of the present invention to provide a peristaltic pump which results in reduced stresses on the fluid-carrying tube and thus longer tube life. It is yet another object of the present invention to provide a peristaltic pump which produces a substantially linear, and non-pulsatile flow for the fluid being pumped. Still another object of the present invention is to provide a peristaltic pump which is relatively easy to manufacture, durable and reliable in its operation and comparatively cost-effective.